LONDON — Britain’s foreign secretary, Boris Johnson, delivered a major speech Tuesday that sought to calm Brexit jitters, wooing “remainers” among the public and the wobbly wing of own Conservative government with promises of the splendid future that is in store.

Twenty-one months after the referendum that launched the country’s departure from the European Union, half the British public remains anxious about or opposed to Brexit, and opponents are clamoring for another vote. Johnson was the first of six top ministers lined up to mount a charm offensive in the coming weeks, making the case yet again for why Britain must go it alone while going global.

On Saturday, Prime Minister Theresa May is expected to give a speech in Munich on the theme of “the road to Brexit.”

British Foreign Secretary Boris Johnson leaves after giving a speech in London on Tuesday as part of a campaign by the government to win public and political support for its road map on Brexit. (Simon Dawson/Pool/Epa-Efe/Rex/Shutterstock)

The roadshow appears to be an acknowledgment of the continuing uncertainties felt at home. The population has watched months of secretive, inconclusive talks in Brussels and roiling debate in May’s cabinet about what leaving actually means: a decisive hard Brexit or a softer, half-in, half-out Brexit with lingering ties to the E.U.’s trade protocols and customs union?

Johnson presented a sunny and optimistic vision of a reawakened global trading giant waving a Union Jack, free to make lucrative deals of its own with the United States and the booming nations of Asia, but only after Britain frees itself from the regulatory shackles and faceless bureaucrats of Brussels.

The witty but often glib foreign secretary, a divisive character at home and abroad, began his remarks by admitting he is frequently the target, in the streets and in his own constituency, of passersby who either hurl invective or express “feelings of grief and alienation” about Brexit.

“They are heartsick at leaving,” he conceded.

They threaten to go to Canada.

Johnson declared that their worries are completely unwarranted. “Brexit is not grounds for fear, but hope,” he said.

But he added a warning of his own: “I fear that some people are becoming ever more determined to stop Brexit, to reverse the referendum vote of June 23, 2016, and to frustrate the will of the people.

“I believe that would be a disastrous mistake that would lead to permanent and ineradicable feelings of betrayal,” he said of the vote, which went 48 percent against to 52 percent for.

“We cannot and will not let it happen,” he said.

The speech drew criticism Tuesday for offering no new details about what May’s negotiators are seeking — or how they intend to wrangle the Europeans into yielding. There was no further clarity on trade and tariffs, on how London’s powerhouse banks will fare or on the fate of Europeans in the U.K. and British citizens residing on the Continent.

Nobody will take away your German wines and Italian cars, Johnson told his audience at a think tank in London.

“It’s not about returning to some autarkic 1950s menu of spam and cabbage and liver,” Johnson said, reminding his older listeners of the grim postwar diet. “It’s about continuing the astonishing revolution in tastes and styles — in the arts, music, restaurants, sports — that has taken place in this country not so much because of our E.U. membership . . . but as a result of our history and global links, our openness to people and ideas that has brought 300 languages on to the streets of London.”

Johnson stressed he has heard the voices of remainers.

“People believe that we have thrown up a figurative drawbridge,” he said. “They fear that the Brexit vote was a vote for nationalism and small-mindedness and xenophobia. They think it was illiberal, reactionary and the British have somehow shown the worst of their character to the world.”

Not so, Johnson said. “I absolutely refuse to accept the suggestion that it is some un-British spasm of bad manners,” he said. “It’s not some great V-sign from the cliffs of Dover.”

Instead, he argued, the vote to leave the European bloc was a vote to take back control. “It is the expression of a legitimate and natural desire for self-government of the people, by the people, for the people. And that is surely not some reactionary Faragiste concept,” a dig at the U.K. Independence Party’s former leader and Trump favorite, Nigel Farage, who was a leading campaigner for Brexit.

Many of the speech’s critics noted its lack of substance, perhaps surprising given that Britain plans to leave the E.U. in just over a year’s time, at the end of March 2019.

Anna Soubry, a Conservative member of Parliament, tweeted that Johnson “fails to understand the very real concerns of British business.”

“Now we urgently need a serious speech that addresses the reality of the practical issues, timescales & contingency planning including Irish border. Time for relentless #OptimismBias is over,” tweeted Sarah Wollaston, a Conservative Party lawmaker.